carbon-based nanomaterials. environmental applications (2).pdf

Development of Multifunctional Nanomaterials and

Adsorption - Photocatalysis Hybrid System

for Wastewater Reclamation

Vipasiri V imonses

B.Sc. (Biotechnology)

M.Eng. (Chemical Engineering Studies)

M.Eng.Sc. (Process Engineering)

Thesis submitted for the degree of

Doctor of Philosophy

School of Chemical Engineering

The University of Adelaide

MARCH 2011

"Success is not final, failure is not fatal: it is the courage to continue that

counts" Sir Winston Churchill-

Table of Contents PhD Thesis: V. Vimonses Page I

TABLE OF CONTENTS

Table of Contents.............................................................................................................I

Abstract...........................................................................................................................X

Declaration..................................................................................................................XIV

Acknowledgement.......................................................................................................XVI

Preface.......................................................................................................................XVIII

Chapter 1. Introduction..............................................................................................1

Chapter 2. Literature Review: Application of Clay Minerals as

Alternative Adsorbents and Photocatalysts for Water

and Wastewater Treatment.....8

1. Background....9

2. Physiochemical Properties and Characterisation of Clay Minerals.........10

3. Modification of clay minerals......14

3.1 Acid/Alkali treatment....14

3.2 Thermal treatment......18

3.3 Surfactant modifications........21

4. Applications of clays in water and wastewater treatment processes...24

4.1 Clay minerals as low-cost adsorbents....24

4.1.1 Adsorption isotherms.....27

4.1.2 Adsorption kinetics and mechanisms.....29

4.1.3 Other key parameters involved in the clay adsorption

performance...31

4.1.4 Organic removal.....34

4.1.5 Dye removal...35

4.1.6 Heavy metal removal..38

4.1.7 Nutrient and inorganic removal..39

Table of Contents PhD Thesis: V. Vimonses Page II

4.1.8 Biological contaminant removal.41

4.2 Clay minerals as catalysts and photocatalyst supports..42

4.2.1 Phocatalysis Mechanisms...44

4.2.2 Immobilisation of photocatalyst on clay supports..46

Chapter 3. Experimental Materials and Methods...72

1. Materials.73

2. Synthesis and Characterisation of Multifunctional Nanomaterials........76

2.1 Formulated Clay - Lime Mixture Adsorbent.....76

2.2 Titania Impregnated Kaolinite Nano-Photocatalyst......76

2.3 Characterisation of the Multifunctional Nanomaterials........77

3. Experimental Procedure......79

3.1 Bench - Scale Operation................................................................................79

3.2 Pilot - Scale Operation...................................................................................80

3.3 An Adsorption - Photocatalysis Hybrid System............................................83

4. Analytical Procedure...........................................................................................84

5. Error Analyses.....................................................................................................87

Chapter 4. Adsorption and Evaluation of Kinetic Isotherm Studies

of Natural Clay Materials.......................................................................89

4.1 Kinetic Study and Equilibrium Isotherm Analysis of Congo Red

Adsorption by Clay Materials.......................................................................90

Introduction..............................................................................................................93

Materials and Methods.............................................................................................94

Materials.......................................................................................................94

Characterisation of Surface Properties of Clay Materials............................94

Adsorption Experiment.................................................................................94

Analytical Method........................................................................................95

Table of Contents PhD Thesis: V. Vimonses Page III

Dye Concentration and Removal Capacity.............................................95

Error Analysis..........................................................................................95

Theory...........................................................................................................95

Adsorption Isotherms..............................................................................95

Adsorption Kinetics.................................................................................95

Adsorption Mechanism............................................................................96

Results and Discussion.............................................................................................96

Characteristics of Clay Materials..................................................................96

Molecular Structure.................................................................................96

Surface Property.......................................................................................97

Effect of Dye Concentration.........................................................................97

Effect of Adsorbent Dosage..........................................................................97

Adsorption Isotherms....................................................................................97

Adsorption Kinetics....................................................................................100

Adsorption Mechanism...............................................................................100

Effect of pH................................................................................................101

Effect of Temperature.................................................................................101

Conclusions............................................................................................................102

4.2 Adsorption of Congo Red by Three Australian Kaolins.............................104

Introduction............................................................................................................107

Materials and Methods...........................................................................................108

Materials.....................................................................................................108

Characterisation of the Kaolins..................................................................108

Experimental Setup....................................................................................108

Analytical Method.....................................................................................108

Table of Contents PhD Thesis: V. Vimonses Page IV

Dye Concentration and Removal Capacity...........................................108

Error Analysis............................................................................................108

Results and Discussion..........................................................................................109

Material Characterisations.........................................................................109

Effect of Initial Dye Concentration............................................................109

Effect of Adsorbent Dosage......................................................................110

Adsorption Isotherms................................................................................110

Adsorption Kinetics....................................................................................110

Effect of pH................................................................................................112

Effect of Temperature.................................................................................113

Material Recovery......................................................................................114

Conclusions............................................................................................................114

Brief Summary of Chapter 4..................................................................................115

Chapter 5. Synthesis and Adsorptive Evaluation of the Calcined Clay

Mixtures and Its Insight Kinetics and Removal Mechanisms.........117

5.1 Enhancing Removal Efficiency of Anionic Dye by Combination

and Calcination of Clay Materials and Calcium Hydroxide...........................118

Introduction............................................................................................................121


Materials.....................................................................................................122

Characterisation of Clay Material Properties.............................................122

Experimental Procedure..............................................................................122

Analytical Method......................................................................................122

Measurement Methods..........................................................................122

Results and Discussion...........................................................................................123

Optimisation of Mixing Ratio and Pre-Treatment Condition

of Clay Mixtures.........................................................................................123

Table of Contents PhD Thesis: V. Vimonses Page V

Optimisation of Clay Mixture Composition..........................................123

Optimisation of Pre-Treatment Activated Condition............................123

Evaluation of Dye Removal Performance by the Clay Mixtures...............124

Effect of Adsorbent Dosage..................................................................124

Effect of Initial Dye Concentration.......................................................125

Kinetic Study.........................................................................................125

Effect of pH...........................................................................................126

Clay Material Recovery and Life Cycle.....................................................126

Conclusions............................................................................................................126

5.2 Insight into Removal Kinetic and Mechanisms of Anionic Dye

by Calcined Clay Materials and Lime.......................................................128

Introduction............................................................................................................130


Materials.....................................................................................................131

Characterisation of Surface Properties of Clay Materials..........................131



Measurement Methods..........................................................................131

Determination of Removal Mechanism Using Isotherm

and Kinetic Models................................................................................131

Error Analysis........................................................................................131


Characterisation of Mixed Materials..........................................................132

Removal Capacity of Clay Mixtures..........................................................133

Adsorption Isotherms and Kinetic Reactions of Clay Mixtures.................133

Adsorption Isotherm Study....................................................................133

Table of Contents PhD Thesis: V. Vimonses Page VI

Kinetic Study.........................................................................................133

Removal Mechanisms............................................................................135

Effect of pH on Removal Mechanisms.......................................................136

Conclusions............................................................................................................136

Brief Summary of Chapter 5..................................................................................138

Chapter 6. Development of a Pilot Fluidised Bed Reactor System with

a Formulated Clay-Lime Mixture for Continuous Removal

of Chemical Pollutants from Wastewater........................................140

Introduction............................................................................................................144


Materials.....................................................................................................145

Reactor Setup..............................................................................................145

Experimental Procedure.........................................................................................145



Dosage Optimisation of the Formulated Clay-Lime Mixture.....................146

Optimisation of Aeration Rate....................................................................147

Removal of Selected Oxyanions by a Fluidised Bed Reactor....................147

Anion Removal in a Continuous Fluidised Bed Reactor System...............147

Determination of the Potential Reaction Time of the Clay-Lime

Mixture..................................................................................................147

Effect of Co-Existing Anions.................................................................148

Continuous Removal of Selected Contaminants from

Primary Wastewater....................................................................................148

Conclusions............................................................................................................149

Chapter 7. Synthesis and Characterisation of Novel Titania

Impregnated Kaolinite and Its Physical Properties and

Photo-oxidation Ability.....................................................................151

Table of Contents PhD Thesis: V. Vimonses Page VII

7.1 Synthesis and Characterisation of Novel Titania Impregnated Kaolinite

Nano-Photocatalyst.......................................................................................152

Introduction............................................................................................................155

Experimental Setup................................................................................................157

Materials.....................................................................................................157

Pre-Treatments of Kaolinite Materials.......................................................157

Preparation of Tio2/K Photocatalysts..........................................................157

Characterisations of Tio2/K Photocatalysts................................................157

Photocatalytic Experiments on Congo Red................................................158


Pre-Treatments of Kaolinite.......................................................................158

Synthesis of Titania Sol..............................................................................159

Heterocoagulation Process..........................................................................160

Heat Treatment of the Photocatalyst...........................................................161

Photocatalytic Degradation of Congo Red.................................................163

Conclusions............................................................................................................163

7.2 Evaluation of the Physical Properties and Photodegradation

Ability of Titania Nanocrystalline Impregnated onto

Modified Kaolin............................................................................................165

Introduction.......................................................................................................167

Materials and Methods......................................................................................168

Materials.....................................................................................................168

Pre-Treatment on Natural Kaolin Clay.......................................................168

Synthesis of TiO2-K Photocatalysts............................................................168

Characterisation of Kaolin and TiO2-K Photocatalysts..............................168

Photoactivity Assessment of TiO2-K Photocatalysts..................................169

Results and Discussion......................................................................................169

Table of Contents PhD Thesis: V. Vimonses Page VIII

Surface Augmentation of Kaolin Clay as Nanocrystal Support.................169

Physicochemical Properties of TiO2-K Photocatalysts...............................170

Variation in the Physicochemical Properties of TiO2-K

Photocatalysts during Repetitive Thermal Regeneration...........................172

Photodegradation of Congo Red Using TiO2-K Photocatalysts.................172

Settleability of the TiO2-K Particles...........................................................174

Conclusions........................................................................................................174

Brief Summary of Chapter 7..............................................................................176

Chapter 8. An Adsorption-Photocatalysis Hybrid Process Using Multi

Functional - Nanoporous Materials for Wastewater

Reclamation........................................................................................178

Introduction............................................................................................................182


Materials.....................................................................................................183

Wastewater Effluent Sources......................................................................183

Experimental Setup of the Adsorption-Phocatalysis

Hybrid Treatment Process..........................................................................184

Fluidised-Bed Reactor System..............................................................184

The Formulated Clay-Lime Mixture Adsorbent........................184

Fluidised-Bed Reactor Setup.....................................................184

Annular Slurry Photoreactor System.....................................................184

Titania Impregnated Kaolin (TiO2-K) Photocatalysts...............184

Annular Slurry Photoreactor (ASP) Setup.................................185




Operation of the Adsorbent Mixture-FBR Adsorption System

Table of Contents PhD Thesis: V. Vimonses Page IX

for Secondary and Tertiary Treatment........................................................186

Contaminant Degradation by the TiO2-K-ASP Photocatalysis System.....188

Chemical Removal Performance of Adsorption-Photocatalysis

Hybrid System............................................................................................189

Photoatalytic Degradation Profile of Dissolved Organic

Pollutants...............................................................................................191

Conclusions............................................................................................................192

Chapter 9. Conclusions and Future Direction......195

1. Conclusions196

2. Major Achievements..197

2.1 Evaluation of the adsorption capabilities of Australian natural clays....197

2.2 Development of a clay-lime mixture as alternative low-cost

adsorbents.198

2.3 Development of a fluidised bed system with clay-lime

mixture for continuous adsorption of wastewater pollutants.198

2.4 Development of a novel titanium dioxide impregnated kaolin

photocatalyst via a reproducible synthesis method...199

2.5 Development of an Adsorption-Photocatalysis Hybrid

System for Wastewater Reclamation.....200

3. Perspective Benefits of the Adsorption-Photocatalaysis Hybrid

System for Water and Wastewater Treatment.......201

4. Future Direction.204

4.1 Modification of the adsorbent mixture...204

4.2 Development of TiO2/K nanophotocatalyst synthesis procedure...204

4.3 Applications of Adsorption-Photocatalysis hybrid system.205

Abstract PhD Thesis: V. Vimonses Page X

ABSTRACT

This thesis study aimed to develop multi-functional nano-catalyst and porous adsorbents

from low-cost and locally available materials, and then implement this into an

Adsorption-Photocatalysis hybrid system for wastewater reclamation. The project

involves two major technological practices for wastewater treatment: adsorption and

photocatalysis. For each technology, a specific functional nanomaterial has been

developed and investigated regarding their removal capability for a pilot-scale water

treatment process. The experimental studies include: 1) evaluation and characterisation

of the natural clay minerals that deliver the most suitable properties for adsorption

performance and immobilisation of titanium dioxide (TiO2); 2) synthesis, modification,

and characterisation of the clay mixtures as alternative adsorbents, and titania

immobilised onto modified porous kaolin as the photocatalyst; 3) evaluation and

optimisation of their removal capability, kinetics and mechanisms toward different

surrogate indicators of both nanomaterials via the batch and continuous water treatment

system; and 4) integration of the adsorption-photocatalysis hybrid system as a major

technical outcome for the treatment and reclaimantion of wastewater.

Three Australian natural clay minerals, bentonite, kaolins, and zeolite, were investigated

to gain understanding of their physiochemical properties as well as their adsorption

capabilities towards Congo red (CR) dye as a chemical surrogate indicator. Microscopic

characterisations revealed the variation of the layered structures among clays, resulting

in the differences in their adsorbent-adsorbate interaction profiles. The removal

capacities of the clays were evaluated through the adsorption isotherms and kinetic

studies, where it was found that Na-bentonite showed the best removal performance,

followed by kaolin and zeolite. Thermodynamic and pH effect studies indicated that dye

adsorption by the studied clays was a spontaneous and exothermic reaction, while pH

conditions appeared insignificant. Further investigation has been emphasised on using

different natural kaolins, in which the recyclability of these clay minerals was also taken

into account. These results depicted a very high thermal stability of the kaolin structure.

Repetitive recycled kaolin trials revealed good recovery of dye removal efficiency even

Abstract PhD Thesis: V. Vimonses Page XI

after five experimental tests. This study demonstrated the potential employment of these

natural clays as alternative adsorbents for wastewater treatment.

To improve the removal efficiency of these natural clays as an economically viable

adsorbent for wastewater treatment, a physical modification of the clay minerals was

adopted in this present work. A feasible technical approach of combination and

calcination of these natural clay materials to improve dye removal efficiency was

developed to compromise the indigenous weakness of individual clays. The application

of a mixture of clay minerals would be able to compromise the indigenous constraints

of the individual clays. An optimisation study using calcium hydroxide or slaked lime

as an additional calcium source for the clay mixture was included. Different

characterisation methods, i.e. differential temperature analysis (DTA) coupled with

thermogravimetric analyser (TG), scanning electron microscopy (SEM), and x-ray

diffraction (XRD), were applied to comprehend the changed properties of the

adsorbents during calcination treatment. The clay mixture and lime showed superior

decolourisation, over 1020 times to those of bentonite, kaolin and zeolite, at the

optimum thermal condition at 300C for 1.5 h. The great enhancement in dye removal

efficiency was the contribution of the combination of an adsorption/precipitation

mechanism. The instant precipitation of dissolved Ca ions with dye molecules

illustrated the major contributor to dye removal, followed by the constant adsorption.

The adsorbent mixture possessed the potential for recovery by heat treatment, of which

their removal capacity was found comparable to the fresh materials even after the 5th

cycle.

The application of the adsorbent mixture was investigated in a pilot scale

implementation, in which the laboratory scale fluidised-bed reactor (FBR) was

developed in our research group. Optimisation of the operating parameters influencing

pollutant removal performance of the FBR system, i.e. adsorbent loading, aeration rate,

reaction time etc. was undertaken to facilitate the continuous operating scheme. The

removal performance of oxyanion phosphate and nitrate in wastewater effluent, as well

as their interference effect on dye elimination was also determined. The results revealed

that the very effective elimination of CR and phosphate as complete removal can be

Abstract PhD Thesis: V. Vimonses Page XII

achieved, while the reduction of nitrate became less extensive due to the difference in

their removal mechanisms, i.e. adsorption and precipitation etc. The feasibility of using

the FBR system in the wastewater treatment was also investigated. Several municipal

primary effluent samples were treated using the FBR system in continuous operation

mode. The results showed an average 10-15% and 20-40% reduction of the nitrate and

chemical oxygen demand (COD), respectively, while 100% phosphate removal was

obtained over the experimental period. This study demonstrated that the FBR system

with the formulated clay-lime mixture can be a cost-effective alternative treatment

process for large-scale application in the wastewater industry.

Another advanced technology, heterogeneous photocatalysis, was used in this study to

improve the quality of treated wastewater. A modified two-step sol-gel method was

developed to synthesise a titanium dioxide impregnated kaolin (TiO2-K)

nanophotocatalyst, in which various parameters affecting the sol-gel formation and

photocatalyst preparation were optimised. Further detailed investigation was carried out

to improve the clay surface function prior to the impregnation. The natural kaolin was

subjected to a series of acidic-alkali treatments to delaminate the clay structure,

followed by thermal treatment. This clay pre-modification was designed to increase the

specific surface area available for heterocoagulation with the microporous titania

particles. Characterisation and photocatalytic activity of the TiO2-K catalyst were

performed by different microscopic techniques, i.e. XRD, SEM, TEM, UV-diffuse

reflectance etc., and CR degradation, respectively. We examined thermal regeneration

cycles of the catalyst lifespan, where the improvement of the photocatalytic activity was

observed as a result of the change in average titania nanocrystal size and their porosity.

This TiO2-K exhibited a superior removal capability over the commercial TiO2 in terms

of initial adsorption and catalyst recovery. The self-settling capability of this catalyst

can facilitate its separation after photooxidation treatment.

Finally, the integration of adsorption and photocatalysis techniques was investigated as

an alternative hybrid system for municipal wastewater treatment. The primary and

secondary biological effluents were preliminary treated by the FBR system with the

synthesised clay-lime mixture before being subjected to an annular slurry photoreactor

Abstract PhD Thesis: V. Vimonses Page XIII

(ASP) using the TiO2-K catalysts. The formulated clay-FBR system demonstrated a

prevailing removal efficiency towards PO43-

, NO3- and suspended solids; whereas the

TiO2-K-ASP showed superior degradation of dissolved organic content. This hybrid

treatment approach demonstrated a synergetic enhancement for the chemical removal

efficiency, and might be able to be employed as a feasible alternative treatment process

for wastewater reclamation.

Thesis Declaration PhD Thesis: V. Vimonses Page XIV

DECLARATION

This work contains no material which has been accepted for the award of any other

degree or diploma in any university or other tertiary institution and, to the best of my

knowledge and belief, contains no material previously published or written by another

person, except where due reference has been made in the text.

I give consent to this copy of my thesis, when deposited in the University Library, being

made available for loan and photocopying, subject to the provisions of the Copyright

Act 1968.

The author acknowledges that copyright of published works contained within this thesis

(as listed below*) resides with the copyright holder(s) of those works.

*List of publications contained in this thesis and copyright holder(s):

1. V. Vimonses, S. Lei, B. Jin, C.W.K. Chow, C. Saint. 2009. Kinetic study and

equilibrium isotherm analysis of Congo Red adsorption by clay materials.

Chemical Engineering Journal. 148: 354-364. Copyright for this paper belongs

to Elsevier B.V.

2. V. Vimonses, S. Lei, B. Jin, C.W.K. Chow, C. Saint. 2009. Adsorption of congo

red by three Australian kaolins. Applied Clay Science. 43: 465-472. Copyright

for this paper belongs to Elsevier B.V.

3. V. Vimonses, B. Jin, C.W.K. Chow, C. Saint. 2009. Enhancing removal

efficiency of anionic dye by combination and calcination of clay materials and

calcium hydroxide. Journal of Hazardous Materials. 171: 941-947. Copyright

for this paper belongs to Elsevier B.V.

4. V. Vimonses, B. Jin, C.W.K. Chow, C. Saint. 2010. Insight into Removal

Kinetic and Mechanisms of Anionic Dye by Calcined Clay Materials and Lime.

Journal of Hazardous Materials. 177: 420-427. Copyright for this paper

belongs to Elsevier B.V

Thesis Declaration PhD Thesis: V. Vimonses Page XV

5. V. Vimonses, B. Jin, C.W.K. Chow, C. Saint. 2010. Development of a pilot

fluidised bed system with a formulated clay-lime mixture for continuous

removal of chemical pollutants from wastewater. Chemical Engineering

Journal. 158: 535-541. Copyright for this paper belongs to Elsevier B.V

6. M.N. Chong, V. Vimonses, S. Lei, B. Jin, C. Chow, C. Saint. 2009. Synthesis

and characterisation of novel titania impregnated kaolinite nano-photocatalyst.

Microporous and Mesoporous Materials. 117: 233-242. Copyright for this paper

belongs to Elsevier B.V.

7. V. Vimonses, M.N.Chong, B. Jin. 2010. Evaluation of the Physical Properties

and Photodegradation Ability of Titania Nanocrystalline Impregnated onto

Modified Kaolin. Microporous and Mesoporous Materials. 132:201-209.

Copyright for this paper belongs to Elsevier B.V.

8. V. Vimonses, B. Jin, C.W.K. Chow, C. Saint. An Adsorption-Photocatalysis

Hybrid System as Alternative Wastewater Treatment by Multi-Functional-

Nanoporous Materials. Water Research. 44: 5385-5397. Copyright for this paper

belongs to Elsevier B.V.

Vipasiri Vimonses

Signed.......................................................... Date......................................................

Acknowledgment PhD Thesis: V. Vimonses Page XVI

ACKNOWLEDGEMENT

I would like to sincerely express my appreciation to the people who have given me

support and encouragement throughout my PhD research journey.

First of all, I am very grateful to my principal supervisor Associate Professor Bo Jin for

his trust and providing me the opportunity to undertake postgraduate research within his

research group. I wholeheartedly appreciate his invaluable support, encouragement and

prompt help in any situations throughout my candidature. Without his guidance, this

thesis would not have been possible. Much appreciation is also dedicated to Professor

Shaomin Lei for his providing guidance on material preparations.

I would also like to express my sincerely gratitude to my industry co-supervisors,

Professors Christopher P. Saint and Christopher W. K. Chow for all their support and

devoting time for guidance and great encouragement for my future career. Their

incomparable knowledge and passion in water industry research and development has

impressed and inspired me towards a professional research career in this area..

I wish to extend my warmest thanks to Professor Rose Amal and the Particles and

Catalysis Research Group members, who previously had given me an opportunity to

work as a research assistant and initiated my interest to carry out PhD research.

Much appreciation also goes to Professor Huaiyong Zhu (QUT), Mr. Philip Adcock

(AWQC), Ms Edith Kozlik (AWQC), Mr. Rolando Fabris (AWQC), Dr Peter Self

(Adelaide Microscopy), Mr Angus Netting (Adelaide Microscopy), Dr Yunfei Xi

(CERAR), for their scientific knowledge and technical support. Without their support,

this project would not have been successful.

Many warm thanks are also dedicated to every past and present members of the Water

Environmental Biotechnology Laboratory for providing a warm and friendly working

atmosphere: Drs. Richard Haas, Zhanying Zhang, Xiaoyi Wang, Yang Yang, Qinxue

Wen, Giuseppe Laera, Hongjie An and postgraduate research students, in particular

Meng Nan Chong, Adrian Hunter, Florian Zepf, Guiqin Cai, Tzehaw Sia, Manjot Kaur,

and all the honours chemical engineering students and visiting German students. Similar

Acknowledgment PhD Thesis: V. Vimonses Page XVII

gratitude is also devoted to the former colleagues in the SA Water Centre for Water

Management and Reuse at the University of South Australia, in particular Professor

Dennis Mulcahy, Dr. John van Leeuwen, Mr. David Pezzaniti, , Ms Carolyn Bellamy,

Ms Julie Wong-Ogisi and most of all, Mr. Tim Golding for his help on the reactor set-

up. I much appreciate their helpfulness and want to thank them for the great time with

joy and laughter that we spent together.

I also wish to thank the University of South Australia and the University of Adelaide for

accommodating my Ph.D. research candidature and providing me with an APAI

scholarship. My acknowledgment also goes to the Australian Research Council and

Australian Water Quality Centre, SA Water Corporation, who contributed substantially

by funding the project.

Finally, I would like to dedicate my achievement and give special thanks to my beloved

parents, and to my dear brothers and sister for their love, care and support along the

way. Also, I wish to express my appreciation to all my friends, I have been fortunate to

have them all to lean on over the years.

Preface PhD Thesis: V. Vimonses Page XVIII

PREFACE

This present project aimed to develop an adsorption-photocatalysis hybrid process for

wastewater reclamation using multifunctional clay materials. This thesis contains nine

chapters, of which Chapter 2, 4, 5, 6, 7 and 8 are the main content. Chapter 1 provides a

general induction and objectives of the project, whilst the general overview of the clay

minerals and their current applications in water and wastewater treatment is illustrated

in Chapter 2. Chapter 3 presents the experimental and analytical methodologies used in

this study. More specific details are also explained in relevant chapters. The research

outcomes and important findings are presented thoroughly in Chapters 4 to 8. Chapter 4

reports physiochemical properties of the natural clay minerals and their adsorption

capacities. This also encompasses microscopic characterisations of the clay materials

and examination of their adsorption behaviours through adsorption isotherms and

kinetic models. Chapter 5 focuses on the development of alternative clay-based

adsorbents that can deliver high removal efficiency towards organic and inorganic

contaminants. A feasible physical approach includes optimisation of clay-lime mixture

composition and their thermally treated condition. The adsorbent mixture demonstrated

a significant enhancement in removal efficiency owning to a combination of

adsorption/precipitation processes as a prime mechanism for dye removal. Chapter 6

describes application of the optimised clay-lime mixture in the mini-pilot scale

operation. A lab designed fluidised-bed reactor (FBR) system was developed. We

studied the influence of different operating parameters on the removal capacity in a

continuous scheme, such as adsorbent loading, aeration rate, adsorbent re-loading time

etc. We examined the removal of nominated oxyanions, i.e. phosphate and nitrate, prior

to the practice of primary wastewater effluent in the FBR system. The obtained result

indicated an excellent removal of anionic dye and phosphate nutrient; while the nitrate

and COD removal was moderately achieved due to a different dominant mechanism.

The following chapter, focussed on an application of the heterogeneous photocatalysis

process for degradation of organic contaminants. A modified two-step sol-gel approach

was used to immobilise layeres of titania nanocrystal onto a core kaolin substrate. Raw

kaolin was preliminarly treated with a series of acid-alkali modifications to promote a

delaminated sandwich silica structure, providing a high external surface area for TiO2

Preface PhD Thesis: V. Vimonses Page XIX

immobilisation. The synthesised titanium dioxide impregnated kaolin (TiO2-K)

nanophotocatalyst demonstrated a superior adsorption capacity, settling ability and

structural stability, in relation to other commercially available TiO2 particles. In Chapter

8, the integration of adsorption and photocatalysis treatment for real wastewater

effluents is reported. The consecutive treatment of adsorption through the clay lime

mixture FBR system, followed by the photooxidation via an annular slurry photoreactor

(ASP) system using the synthesised TiO2-K nanocatalyst was investigated. The

adsorption-photocatalysis hybrid system revealed a synergetic enhancement for the

contaminant removal efficiency. Complete elimination of phosphate content was

obtained in the adsorption stage; while moderate nitrate removal was obtained from the

hybrid treatment. The corresponding COD reduction during the photodegradation was

further investigated by the advanced high performance size exclusion chromatography

technique, where it revealed the shift of apparent molecular weight of the dissolved

organic contaminants toward the smaller molecular weight region. This present study

demonstrated that this adsorption-photocatalysis hybrid technology can be used as a

feasible alternative treatment process for wastewater reclamation. Lastly, the results and

conclusions from each individual chapter are given, in conjunction with further

discussions of the future perspectives for continued work in this area.

This thesis has been prepared as a series of publications, of which Chapters 4, 5, 6, 7

and 8 have been published in referred international academic journals. The remaining

Chapter 2 will be submitted for publication in a refereed journal. All the publications

are closely relevant to the research area of this present work.

CHAPTER 1

INTRODUCTION

Chapter 1 PhD Thesis: V. Vimonses Page 2

1. Background

The global population has grown significantly over the last century, this has lead to a

great increase in the demand for water supply. The water shortage situation could

develop into a global water crisis. Reliance on surface water bodies alone seems to be

insufficient to respond to this rising demand, while heavy extraction of groundwater can

lead to negative long term effects such as land subsidence. Seeking new reliable water

sources, therefore, has turned into an urgent environmental issue. Treated wastewater is

one of the promising alternatives as a reclaimed water source, which has drawn an

interest over the last few years, especially in arid and semi-arid regions where water

sources are limited. The treated wastewater is a readily available and reliable source,

and recycling would reduce the extensive amount of water extracted from the

environment [1]. The potential use of the treated wastewater can be varied significantly,

depending upon the degree of treatment and more importantly on public acceptance.

This practice has been carried out in many countries especially for irrigation purposes,

e.g. agriculture, landscapes, and recreational areas [2].

Water quality is a crucial issue for reuse of the treated wastewater. Health hazards are

the major restriction for the reuse of wastewater which leads to a higher stringency of

the water quality requirement. Generally, the treated wastewater must at least meet

minimum safety standards for specific reclaimed purposes. In general, conventional

treatment processes, including primary and secondary treatments, are able to remove

over 90% of chemical contaminants and up to 95-99% of microorganisms. However,

direct reuse of this effluent cannot be practised without further treatment due to the

presence of high organic content and pathogenic organisms [3-4]. The effluent from

secondary treatment still contains some suspended, colloidal, and dissolved constituents

as well as a variety of pathogens which are hazardous. Hence, consecutive tertiary

treatment and disinfection processes are required to ensure the exclusion of pollutants,

and that the water obtained is harmless. Traditional tertiary treatment such as deep

filtration, macro-filtration and typical adsorbents can only partly remove the residuals,

while the remaining pathogens are passed to the disinfection stage for final removal. To

date, the currently available treatment technologies regarded as effective processes for


reclaiming the municipal wastewater effluent are still experiencing a series of technical

and economical challenges. This has lead to extensive research of advanced

technologies that can overcome such inherent limitations.

Nanotechnology has become one of the most significant technologies of the 21st

century. It encompasses a broad range of tools, techniques and applications based on a

structure size between 1 and 100 nm. A unique aspect of nanotechnology is the

enormously increased ratio of surface area to volume presented in many nanoscale

materials, leading to new possibilities in surface-based science. Due to their small size

and well-organised structure, nanoscale materials offer an alteration of physio-chemical

properties of the corresponding bulk material properties, e.g. colour, strength, and

thermal resistance etc., providing opportunities to be exploited in many industrial facets.

Nanomaterials are considered as new functional materials used in industry based

techniques and processes, such as cleaning-up industrial contamination, and improving

energy production and uses.

Water purification is identified as a priority area for nanotechnology applications due to

readily available appliances incorporating nanomaterials, and the public pressure for

clean water requirement [5]. High quality treated water can be produced from this

innovative technology for reuse purposes and with the benefit of not generating

disinfection by-products. Over decades, a wide range of water treatments incorporating

nanosciences have been studied such as nanofiltration membranes, nanoporous/filter

materials, nanocatalysts, magnetic nanoparticles and nanosensors. Many technology

developers stated that these nanotechnologies offer more cost-effective processes for

removal of aqueous pollutants in wastewater [5].

Nanoporous materials are considered to be an alternative resource offering potential for

the wastewater treatment industry. They show a specific capability in the removal of

hazardous substances either through adsorption or size exclusion. The presence of

nanopores facilitates their important uses in various fields such as ion exchange,

separation, catalysis, sensing, biological molecular isolation and purification. The major

advantages of using these materials over conventional or advanced membrane filtration

are due to their low-cost and abundant availability, lower energy requirement for


operation, and the capability of handling wastewater with high organic content. A

number of publications have addressed the utilisation of nanoporous materials for

removal of various pollutants such as organic and inorganic compounds, heavy metals,

microorganisms etc. In some cases, the polluting adsorbates can be ultimately

decomposed through thermal adsorbent regeneration. In addition to this, the

applications of nanoporous materials have been recently extended into catalytic

oxidation of water pollutants, in particular for degradation of recalcitrant or non-

biodegradable compounds. The materials are commonly used either as an active

catalyst itself or as a supported substrate due to their high surface area. It is suggested

that the introduction of nanoparticles in heterogeneous catalytic processes can

appreciably enhance the catalytic efficiency [6].

Among available nanoporous materials, natural clay minerals have received

considerable attention as alternative lowcost materials due to being non-toxic,

abundant, environmentally friendly and possessing multifunctional properties

depending upon the types of clays. Zhou et al [7] suggested that the nano-structural

layer and nano-sized layer space of the clay minerals can act as naturally occurring

nanomaterials or as nano-reactors for production of nano-species, nanoparticles or

nanodevices. However, in many instances, inherent downsides of the natural clays such

as high impurities, heterogeneous functional surfaces, fragile molecular structure, as

well as favourable interaction toward particular compounds etc, has limited their uses to

certain applications. Such drawbacks can be minimised by adopting a physiochemical

modification prior to utilisation, i.e. chemical grafting, sonication, mechanical grinding,

acidalkaline treatment, thermal activation etc.

Nevertheless, selecting appropriate types of clay together with suitable modification

methods is considered as one of the technical challenges. Variation of the clay structures

based on their types and origins could result in different performances and behaviours

toward the treatments. Therefore, it is essential to gain in-depth information of the clay

characteristics and their adsorption properties toward a targeted compound. This leads

to a requirement of the adsorption isotherms and kinetic studies, where the removal

performance and mechanisms of the clays can be determined. Furthermore, the


significant effects of influential systematic parameters on the adsorption efficiency such

as solution pH and temperature are also observed in many cases.

Despite being used as alternative adsorbents, the application of the clay and modified

clays in the area of Advanced Oxidation Processes (AOPs) for water and wastewater

treatment has gained much attention. The AOPs principle is based on the in-situ

generation of highly reactive radicals (i.e. OH, O2

- etc.) for oxidation of recalcitrant

organic compounds and pollutants, pathogens and disinfection by- products [8-10].

Among these, heterogeneous titanium dioxide (TiO2) photocatalysis in a slurry system

(suspension of fine powdered TiO2) has been widely studied, owing to its potential to

mineralise a range of refractory organic pollutants at ambient temperature and pressure

into innoxious substances [11]. However, a separation of the fine TiO2 catalysts after

treatment can be energy intensive and time consuming, resulting in a significant

reduction in the benefits of this technique in water treatment industries. This technical

constraint leads to development of an immobilised photocatalyst on an inert support to

avoid costly separation processes after treatment. The titania immobilisation process

often involves several intense chemical and thermal interactions, hence the clay porous

materials that possess a highly stable structure are often in great demand as catalyst

supports.

Other technical challenges commonly facing such nanotechnological applications in

large scale wastewater treatment are the difficulty in process implementation and

recovery of the materials after treatment. Since small size particles are often employed

to deliver high adsorption/removal efficiency, the operating procedure can be

complicated. In terms of engineering aspects, it is believed that such problems can be

overcome by a well-designed reactor and optimised operating conditions. The

appropriate design of an operating module incorporating proper arrangement of

treatment processes could offer a better potential to deal with complicated wastewater

sources. In this present work, an integration of adsorption and photocatalysis processes

using an in-house developed multifunctional nanoporous material will be investigated to

achieve a high quality of treated wastewater for reclamation.


2. Aims

The aim of this study was to develop multifunctional nanomaterials and an effective

hybrid treatment system that could be employed as an alternative tertiary wastewater

treatment process for reclaimed wastewater.

The project involves two major technological practices for wastewater treatment, i.e.

adsorption and photocatalysis, which encompass four separate research and

development stages. These include:

(1) Evaluation and characterisation of the natural clay minerals that deliver the most

suitable properties for adsorption performance and immobilisation of titanium

dioxide (TiO2);

(2) Synthesis, modification, and characterisation of the clay mixtures as alternative

adsorbents, and titania photocatalyst immobilised onto modified porous kaolin;

(3) Evaluation and optimisation of both nanomaterials for their removal

capabilities, kinetics and mechanisms toward different surrogate indicators such

as industrial dye and sewage wastewater via the batch and continuous water

treatment system;

(4) Integration of the adsorption-photocatalysis hybrid system as the best available

technical solution to treat reclamation wastewater.

3. References

[1] S. Toze, Reuse of effluent water benefits and risks. Agric. Water Manage. 80

(2006) 147-159.

[2] P. Cornel, B. Weber, Water reuse for irrigation from wastewater treatment plants

with seasonal varied operation modes. Water Sci. Technol., 50 (2004) 47-53.

[3] H.K. Shon, S. Vigneswaran, S.A. Snyder, Effluent organic matter (EfOM) in

wastewater: Constituents, effects, and treatment, Crit. Rev. Env. Sci. Technol. 36

(2006) 327-374.

[4] M. Gmez, A. de la Rua, G. Garraln, F. Plaza, E. Hontoria, M.A. Gmez, Urban

wastewater disinfection by filtratin technologies. Desalination, 190 (2006) 16-28.


[5] T. Hillie, M. Munasinghe, M. Hlope, Y. Deraniyagala, Nanotechnology: Water and

Development, Global Dialogue on Nanotechnology and the poor: Opportunities

and Risks, Meridian Institute, www.merid.org, accessed on 30 June 2006.

[6] E.G. Garrido-Ramrez, B.K.G Theng, M.L. Morac, Clays and oxide minerals as

catalysts and nanocatalysts in Fenton-like reactions:A review, Appl. Clay Sci. 47

(2010) 182-192.

[7] C.H. Zhou, D.S. Tong, M.H. Bao, Z.X. Du, Z.H. Ge and X.N. Li, Generation and

characterization of catalytic nanocomposite materials of highly isolated iron

nanoparticles dispersed in clays, Top. Catal. 39 (2006) 213219.

[8] S. Esplagus, J. Gimnez, S. Conteras, E. Pascual, M. Rodrguez, Comparison of

different advanced oxidation processes for phenol degradation, Water Res. 36

(2002) 1034-1042.

[9] M. Pera-Titus, V. Garia-Molina, M.A. Baos, J. Gimnez, S. Esplagus,

Degradation of chlorophenols by means of advanced oxidation processes: A

general review. Appl. Catal. B: Environ. 47 (2004) 219-256.

[10] H.M. Coleman, C.P. Marquis, J.A. Scott, S.S. Chin, R. Amal, Bactericidal effects

of titanium dioxide-based photocatalysts. Chem. Eng. J. 113 (2005) 55-63.

[11] M.E. Fayibi, R.L. Skelton, Photocatalytic mineralisation of methylene blue using

buoyant TiO2-coated polystyrene beads, J. Photochem. Photobiol. A 132 (2000)

121-128.

CHAPTER 2

LITERATURE REVIEW

APPLICATION OF CLAY MINERALS AS ALTERNATIVE

ADSORBENTS AND PHOTOCATALYSTS FOR WATER

AND WASTEWATER TREATMENT


1. Background

Clays and clay minerals have been widely utilised for many industrial practices, such as

in geotechnology, agriculture, construction, engineering, process industries, and

environmental applications [1-2]. The major utilisation of the clay minerals can be

classified into two contrasting broad classes, i.e. based on their inertness and stability,

and their reactivity and catalytic activity [3].

Among natural materials, clay minerals (aluminosilicates) have several important

advantages over alternative adsorbents. They are inexpensive, abundantly available and

non-toxic, and have good sorption properties and ion exchange potential for charged

pollutants. They possess a wide pore size distribution, ranging from micro- (


minerals in water and wastewater treatments will be focused, in particular as alternative

low-cost adsorbents and catalytic supports for photocatalytic degradation, which are the

objective of this study.

2. Physiochemical Properties and Characterisation of Clay Minerals

Depending upon the geographic source of origin, the physiochemical properties of clay

minerals can be varied and is the main determining factor for their utilisation. The term

clay is generally used to refer to aluminosilicate minerals whose particle sizes fall into

a micron range, and possess cation-exchange properties [16]. With variety in the

structural orientation, clays can be categorised into different groups. A phyllosilicate or

sheet silicate is one of the most important groups for industrial applications, and is

generally termed as clay minerals. The basic structure of the clay minerals consists of

a tetrahedral sheet of polymerised silica and octahedral sheet of alumina. The alumina

octahedra can polymerise in two dimensions by sharing four oxygen atoms, in which

two oxygen atoms are left unshared, providing a negative charge of two. This negative

charge is counterbalanced by hydrated cations, e.g. Na+, Mg

2+, Ca

2+ etc., which are

located in the interlamellar space. Such interlamellar cations are typically exchangeable

and their amount indicates the cation-exchange capacity (CEC) of the clay minerals

[17]. Due to their extremely fine particles, the clay minerals tend to exhibit the chemical

properties of a colloid [18-19]

In general, clay minerals are identified according to their magnitude of net layer charge,

layer structure arrangement, and interlayer species [20]. The clays are placed alternately

by tetrahedral and octahedral sheet structures, which are classified into different layer

types. Such structures are distinguished by the number of tetrahedral and octahedral

layers [19]. Diversities in such layered structures are categorised into different classes

e.g. smectites (montmorillonite, saponite), mica (illite), kaolinite, serpentine,

pylophyllite (talc), vermiculite and sepiolite [21]. The most common clay minerals used

in the water and wastewater industries are classified into 3 different types, i.e. 2:1 and

1:1, and zeolite groups, which will be employed in this study.


The 2:1 type clay minerals consist of two silicon-oxygen tetrahedral sheets that

sandwich an aluminium-oxygen-hydroxyl tetrahedral sheet (Fig.1a). There are three

main mineral groups possessing this structure, i.e. illite, vermiculite, and smectite [22-

23]. The isomorphous substitution of Al3+

for Si4+

in the tetrahedral sheet and Mg2+

or

Fe2+

for Al3+

in the octrahedral sheets could result in a negative surface charge on the

clay. These charges are permanent and less subjective to pH changes, yet such pH

sensitivities do vary with the type of the host clay [19]. This unbalanced charge can be

compensated by exchangeable cations, i.e. Na+ or Ca

2+, which are relatively loosely

held, giving rise to cation exchange properties. A second negative charge takes place as

a result of dissociation of the hydrogen in various hydroxyl groups, including SiOH and

AlOH groups, of which the dissociated propensity increases with an increase in pH.

Another source of negative charge of the clays can be owing to their low coordination

number edge atoms; these charges can be neutralised by compensation cations [19].

Such a layer structure of the clay allows expansion (swelling property) when it contacts

water, which provides an additional mineral surface for cation adsorption [19].

Examples of common 2:1 clays used in chemical and industrial applications are

montmorillonite and bentonite. Montmorillonite possesses a very large surface area, and

hence has been the subject of most studies involving clay nanocomposites [24].

Bentonite can be divided into different types based on the dominant exchangeable

cation, i.e. Na-bentonites and Ca-bentonites. It acquires unique properties, such as large

chemically active surface area to volume ratio, a high cation exchange capacity and

inter-lamellar surface possessing unusual hydration characteristics [25]. These features

allow them to be modified by a range of different surface treatments such as organoclay

[26], acid-alkali treatment [27] and grafting polymer [28]. The use of montmorillonite

and bentonite based materials in water treatment have been widely reported, i.e.

removal of Cr(VI) [26,29], Pb (II) [30], industrial dyes [8,12,31] , coagulation [32],

flocculation [33] etc.


Figure 1: A diagram of clay mineral molecular structures a) Bentonite

(Montmorillonite), b) Kaolinite and c) Zeolite [8]

In contrast, the 1:1 clay mineral refers to clays with a structure comprising as

alternative layers of a tetrahedral silica sheet and an octahedral alumina sheet, where the

layers are bound together by hydrogen bonds. Kaolinite is one of the most commonly

known 1:1 clay minerals, which is classified as a phyllosilicated (sheet silicates) mineral

with the chemical composition of Al2Si2O5(OH)4. The layers are tightly held together to

form clay stacks with a basal spacing of around 7.1A [34]. The lattice structure of

kaolinite is nearly in perfect order and thus there is relatively low isomorphous

substitution, leading to a limited swelling of kaolinite in water (Fig. 1b). Such ordered

arrangement of the clay gives the kaolinite a rigid and chemically and thermally stable

structure with a low expansion coefficient. Thus, most sorption activity tends to take

place along the edges and surfaces of the clay structure [35].

Zeolite is very similar to the clay minerals in its composition, i.e. hydrated

aluminosilicates, however they differ in their crystalline structure. This inorganic

polymer represents the largest group of microporous materials, which have a high

+ -

O

Si

Al

H

H2O

Cation

H bond

a) c) b)


adsorptive selectivity feature. Zeolite is built by connecting SiO2 and AlO4 tetrahedra in

a network formation of a cage-like structure as shown in Fig. 1c.

Figure 2: Framework structures of zeolites based on sodalite cage as a secondary

building block [36]

Fig. 2 demonstrates the different framework structures of zeolites based on the sodalite

cage as a secondary building block [36]. Their symmetrically stacked alumina and silica

tetrahedral sheets lead to an open and stable three dimensional honey comb structure

with a negative charge interconnected cages and channels with Brnsted and Lewis acid

sites [19, 37-38]. These negative charges within the pores can be neutralised by

positively charged ions, i.e. Na+, which are exchangeable with certain cations in

solutions such as heavy metals and ammonium ions [22, 39]. Variation of functional

properties of zeolites enables them to be employed as adsorbents, molecular sieves,

membranes, ion exchangers and catalysts in industrial applications for water/wastewater


treatment, gas separation and dehydration processes [40]. However, the major use has

been found in water and wastewater treatment [41].

Aluminosilicate zeolites are highly hydrophilic adsorbents owning to their electrostatic

charged framework and abundance of extra-framework cations. Other factors

contributing to their unique hydrophilicity include defect sites, surface nature, metal

constitutions in the framework, and the amount of coke deposits [36]. The zeolites with

medium and large pores are often used in catalysis to facilitate the diffusion of

molecules reaching the catalytic active sites in the material pores. In contrast, those with

a high concentration of cation exchange sites and small pores are more suitable for

adsorption processes due to the molecular sieving effect. To date, there are more than 40

species of natural zeolites (with more than 100 species for synthetic ones) [42]. Among

these, clinoptilolite, a mineral of the heulandite group, is the most abundant and

frequently studied [43], and has been widely used for many industrial applications [14].

A number of researchers have evaluated the capacity of zeolites in removal of

methylene blue dye in water [44], phenols and chlorophenols [45], and geosmin and

methylisoborneol from drinking water [46]. In addition, the chemical and cage-like

structural characteristics of zeolites make them very effective to eliminate toxic metal

ions, e.g. Pb2+

[47], radionuclides and ammoniacal nitrogen from wastewater [41,43].

3. Modification of clay minerals

The physiochemical properties of clay minerals can vary, depending upon the

geographic source of origin, and is the main factor determining their ultimate utilisation.

These properties can be altered by several modification methods to enhance their

functional ability and efficiency. Such alterations have been addressed in many

publications and are summarised below.

3.1 Acid/Alkali treatment

One of the most common chemical methods for clay modification is known as acid

activation. This modification involves the treatment of clay with concentrated inorganic

acid, i.e. HCl or H2SO4. This process results in an increase in the clay specific surface


area, porosity and surface acidity [48]. Important parameters for the acidification, which

determine the properties of the products, include the nature and type of clay, acid

concentration, temperature and activation time, etc. [17, 49]. In addition, this acid

activation is believed to promote catalytic activity of the clay by increasing the number

of Brnsted and potential Lewis sites.

Komadel and Madejov [50] stated that the acidification of clay minerals under

controlled acid conditions can induce partial dissolution of other elements (i.e. Fe and

Mg) within the clay structure. Such an acid attack preferentially occurs at the octahedral

alumina sheet, while the tetrahedral sheet (i.e. SiO4 and SiO3OH) remains largely intact

[51]. This dealumination leads to an increase in the silica to alumina ratio. The opposite

was observed in alkali treatment which also resulted in dissolution of the most irregular

(amorphous) mineral particles [52]. This alteration was confirmed by X-ray diffraction

(XRD) diagrams where they showed severe destruction of the crystal lattice after acid

treatment; while sharpening of the XRD reflection after alkali treatment was observed.

A similar phenomenon was also addressed by Gupta and Bhattacharyya [53] where they

demonstrated that the intensities of the characteristic XRD peaks can reduce upon

acidification. Many studies observed the increase of porosity in the tens of nanometres

range after acid treatment [54], in which these pores could be formed as cavities on

basal or edge surfaces of the clay particles or as a result of a decrease in particle sizes

due to partial dissolution of larger particles. The adsorption studies of the activated

montmorillonite demonstrated a considerable improvement in adsorption capacity

towards diazo acid dyes such as amido black [55] and methylene orange [56] after

acidification by HCl. The increase in its adsorption capacity was due to the substitution

of the Al3+

or Fe2+

of the clay by H+ after acidification demonstrated by the specific

surface area (BET) and XRD analysis. Chemical treatment under acid and alkali

conditions can result in remarkable changes in pore behaviour of the clay minerals. It is

recognised that the alteration can be more prevalent in nanosize pores than larger pores

[54].

In general, both acid and alkali treatments can increase variations in surface charge,

while the cation exchange capacity (CEC) is altered depending on the types of minerals

and treatment methods. Acidification leads to an incremental increase in the amount of


weakly acidic surface functional groups associated with a decrease in the amount of

groups of stronger acidic character. Unlike alkali treatment, the amount of surface

groups of intermediate acidity increases and that of low acidity decreases [57].

Jozefaciuk and Matyka-Sarzynska [27] studied the effect of acidification and

alkalisation on nanopore properties of bentonite, biotite, illite, kaolin, vermiculite and

zeolite by well-defined adsorption-desorption isotherms, where they found an increase

in specific surface areas of the minerals under both conditions. However, Komarov [58]

suggested that the impacts of acidic treatment were more pronounced for

montmorillonite, whilst negligible change was observed in the case of kaolinite.

Guo et al [59] prepared decolourising ceramsites via acid-alkali treatment with H2SO4

and NaOH, followed by the thermal activation from different clays minerals, i.e.

sepiolite, bentonite and palygorskite etc. They observed that clays possessing high

Al2O3, Fe2O3 and MgO content demonstrated the best performance for dye and organic

removal. This study also demonstrated the preliminary basic treatment followed by acid

activation was found to enhance the overall decolourising performance through

chemical flocculation with precipitated metal ions from the adsorbent.

The changes of surface and morphological properties of the treated clay after chemical

modification can be examined via scanning electron microscopy (SEM) imaging. Melo

et al [24] observed the surface alteration of kaolinite after chemical treatment with H2O2

and acid modification (Fig.3). They observed that the micro-size particles of the

untreated clay are composed of individual platelets conglomerating into larger size

particles. After treatment with H2O2, a separation of some discrete platelets was

evidenced. This can be explained by the removal of organic matter, which contributes to

a reduction of the agglomerating effect. Such chemical treatment also was found to

increase the specific surface from 35.3 to 53.0 m2 g

-1.


Figure 3: SEM images of a) untreated kaolinite, b) kaolinite after chemical treatment

with H2O2, and c) kaolinite after chemical treatment using H2O2 and acid acidification

with HNO3 and H2SO4 [24]


3.2 Thermal treatment

Calcination is another well-known treatment method for the activation of clay

adsorbents. Heating can bring about the modification of structure, physiochemical

properties, and composition of the clay minerals, including strength, cohesion,

consistency limits, water content, maximum dry density, internal friction angle, particle

size, permeability and specific gravity etc. [60]. Additionally, the thermal treatment can

assist stabilisation of the clay to maintain important permanent properties. These

concomitant changes can be varied from one clay mineral group to the other, as well as

the particle size and heating regime. Raising the temperature to dehydration stage leads

to a loss of adsorbed water causing alteration of the macro- and microporosity of the

clay minerals, resulting in the collapse of interlayer spaces and reduction of cation

exchange capacity (CEC). Partial loss of the adsorbed and hydration water can increase

hydrophilicity and surface acidity of the materials [61]. It is often that the heat treatment

is preceded by other treatment methods, i.e. chemical modifications, to achieve better

performance.

Heller-Kallai [61] suggested that thermal treatment of the clay materials can be carried

out in different forms: i) without any admixtures or pre-treatment; ii) mixed with

various reagents before heating, iii) after pre-treatment, i.e. acid activation; and iv) after

pre-heating and pre-treatment, i.e. by acid activation and subsequent re-heating. The

degree of thermal tolerance can vary depending upon the clay types. It is suggested that

dehydration at a high temperature can cause the irreversible collapse of the structure.

The clay platelets are bonded electrostatically. Therefore, dehydration of cations can

result in a reduced adsorption ability of the clay [36]. For instance, the study by Joshi et

al [61] revealed a gradual increase in the strength of the studied clays (kaolinite and

bentonite), and clay sediment with an increase in heating temperature. Yet, such

significant and permanent increase in their strength occurred only after dehydroxylation.

By continuing heating to a temperature above the dehydroxylation stage, the clays

became resistant to disintegration upon immersion in water. Thus, thermoanalytical

measurement of the clay minerals is often performed to understand the stability of their

structure against an elevated temperature gradient. The thermogravimetry/differential

thermal analysis profiles of natural Na-bentonite, kaolin and zeolite are given in Fig. 4


[63]. It was found that the kaolin demonstrated the highest structural stability due to the

strong bond between its aluminosilicate layers. In case of zeolites, dehydration at high

temperatures (>200 C) is observed as a result of a strong interaction between the

electrostatic charged framework with the water molecules [36].

In addition to this, an improvement in adsorption performance of the calcined clays was

also reported in many publications. Guo et al [59] found that the decolourising of dye

manufacturing effluent by ceramsites can be improved two-fold by raising the

calcination temperature from 300 to 700 C.

The effects of thermal and chemical treatments on physical properties of kaolinites were

also recently investigated by Melo et al [24]. Modification of the clay structure after

heat treatment at 500 C for 5 h was evidenced by the changes of the XRD pattern,

where they observed the formation of an amorphous phase in the thermally treated clay.

This is related to the loss of hydroxyl groups, which cause a rearrangement of the

original structure of kaolinite. Vieira et al [64] evaluated the morphological changes of

the calcined bentonite and its adsorption efficiency toward nickel in the porous bed. The

clay was treated at 500C for 24 h to increase its mechanical resistance and to eliminate

some impurities. They reported that the calcination led to an increase in the clay surface

area and the development of micro and mesoporosity due to bonded water release and

dehydroxylation.. This can cause a reduction in clay density compared with the

untreated one; while the adsorption of nickel in clay pores resulted in a slight increase in

actual density.


Figure 4: Differential thermal analysis (DTA- Heat flow against temperature profile) and thermogravimetric (TG- Weight loss against

temperature profile) profiles of a) Sodium Bentonite, b) Kaolin, and c) Zeolies [63]

a) c)

TDA TDA TDA

TG TG TG

b

)


3.3 Surfactant modifications

In general, natural clays posses electrically charged and hydrophilic surface

characteristics surface due to isomorphous substitution in their crystal lattice, and thus

they are very efficient adsorbers of ions and polar molecules [65]. The application of

clay for removal of non-ionic organic pollutants, on the contrary, is likely limited.

Surface modification using functional polymers is one of the most effective methods to

alter the surface properties. This surface modification of the clay minerals is generally

carried out by two main approaches: physical adsorption or chemical grafting of

functional polymers to the surfaces of the clays [66]. The surface of clay minerals can

be changed via the physical adsorption of specific functional polymers, resulting in

improving the physical and chemical properties without rendering their structure. This

physical attachment is usually controlled by thermodynamic criteria and therefore

results in weak forces between the adsorbed polymers and the clay minerals, i.e. Van

der Waals forces.

In contrast, chemical grafting of functional polymers can render the ability to control

and alter the properties of clay mineral surfaces. The grafting method allows the

polymer chain to be tightly bound onto the clay surfaces by covalent bonds via the

condensation of functionalised polymers with reactive groups of the clays [66]. The

grafting procedure can be carried out through different pathways. For instance, in case

of 2:1 clay minerals, guest compounds can be intercalated from the vapour, liquid, and

solid state [67]. Saline coupling agents are commonly used to prepare micro-composites

based on clay, silica and fibre glass etc [68]. Once the clay mineral is grafted with

organosilane, their hydrophilic surface becomes organophillic and can be easily

dispersed in low-polarity compounds including polymers [69]. Shen et al [70] studied

the grafting mechanism of saline onto montmorillonite for environmental remediation.

They found that the grafting reaction from the silane vapour favoured intercalation of

silane into the clay as compared to that of the aqueous phase. Recently, Wang et al [71]

introduced a new grafting method, where polymer chains were grafted onto the surface

of the clay silicate layers via high-dose gamma-ray irradiation. This method was

suggested to simpler compared to conventional ones.


Clay surfaces modified with an organic ion surfactant are known as organoclay. This

surfactant modification enables the transformation of organophilic surfaces to

organophobic surfaces, and therefore the adsorption interaction towards organic

compounds can be enhanced. The organoclay consists of alternate organic and

inorganic layers. The organic layer, or the hydrophobic layer, will act as an organic

phase for the reactable dissolved organic substances. The surface modification can be

preceded by insertion of alkyl-ammonium cations or quaternized cationic surfactant into

the interlamellar spacing to create a sorption zone for hydrophobic contaminants [72-

73]. Characteristics of this zone can be altered using various surfactants, e.g.

replacement of exchangeable cations with organic cations which have long-chain alkyl

groups [74], a number of positive charges [75], structural aspects (number of alkyl

branches, presence of aromatic moieties), and polarity index of the alkyl groups [76].

Through hydrophobic binding, the clay-organic complexes are efficient in adsorbing a

variety of organic molecules from water.

The application of organoclays in adsorbing organics was first recognised by McBride

et al [74]. They investigated the adsorption of 2,4-D compounds on Bentone 24, a

montmorillonite ion exchanged with dimethyl benzyl octadecyl ammonium ions. Since

then, the modified organoclays have been applied in a wide range of practical cases for

organic pollution control, such as water purification [77], industrial wastewater

treatment and remediation of contaminated ground water [78]. It is suggested that the

organoclay can outweigh the conventional activated carbon in some circumstances, as

the activated carbon is incapable of removing large molecules such as humic acid [79],

and emulsified oil and greases [80]. Numerous studies have focused on using bentonite

as a substrate for organoclay syntheses [12,81-82]. zcan et al [12] introduced a large

organic cation surfactant onto Na-bentonite to produce a Dodecyltrimethylammonium

bromide-modified bentonite (DTMA-bentonite), which showed significant

improvement for removing acid dye, which was approximately 11 times higher than the

original clay. The adsorption efficiency of the organoclays for removal of organic

pollutants in water has been evaluated and reported in many publications [83-84].

In addition to this, adsorption mechanisms of the organoclays towards organic

compounds and various methods for improving their adsorption capacities have been


extensively studied and proposed [85]. It is suggested that the degree and mechanism of

the adsorption is greatly dependent on the molecular structure of the organic cation that

is used to modify the clay surface [81].

Barlelt-Hunt et al [81] and Shen et al [86] proposed that if the organoclays are prepared

with small organic cations, the contaminated organic compounds tend to be primarily

adsorbed onto the hydrophobic siloxane surface of the modified clays. The adsorption

capacity of this class of organoclays is enhanced by increasing the exposed siloxane

surface [86-87].

The uses of natural clays as coagulation aids for improving the settling performance

have been long known [77, 88-89]. Recently, the application of organoclays for

coagulation treatment was also investigated. Jiang et al [90] developed a polymeric

carbon-based nanomaterials. environmental applications (2).pdf

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